This dataset indicates the likelihood of inadequate percolation for Domestic Waste Water Treatment Systems (DWWTS) based on mapped geological information. This dataset was developed as part of a risk-based methodology to assist in the regulation of Domestic Waste Water Treatment Systems (EPA, 2013). http://www.epa.ie/pubs/reports/water/wastewater/dwwtriskranking.html. The aim of the report was to set out a methodology to enable the EPA to adopt a risk-based approach to organising inspections of domestic waste water treatment systems (DWWTSs), whereby the level of inspection would be proportionate to the risk posed to human health and the environment.